The isokated rat liver perfused for periods of 12 or 24 hours continues to be used as a model system for the study of factors regulating the net biosynthesis of specific plasma proteins; these include albumin, fibrinogen, alpha 1 acid glycoprotein (Darcy), and alpha 1-acid glycoprotein (Kawasaki), alpha 1-macroglobulin, alpha 2-(acute phase) globulin, haptoglobin, hemopexin, antithrombin III, C3-component of complement, alpha2-Hs-globulin and transferrin. Studies of hormones are to be extended to include androgens and estrogens as well as bradykinin and vasopressin. Concomitant changes in protein synthesis and functional mRNA will be quantitatively estimated to define the role of the modulation of mRNA as an important mode of regulation of protein synthesis; such charges will be produced by experimental injury in the liver donor or during perfusion by inhibitors of mRNA or protein synthesis.